Mr imaging method

ABSTRACT

The invention relates to an MR imaging method in which at least one gradient magnetic field which traverses the object ( 7 ) to be examined is applied for a given period of time, a co-ordinate system being associated with the object ( 7 ) to be examined. In most cases the parameters of the MR method are already determined by the medical task. It is an object of the invention to provide an MR method which offers, without having to sacrifice imaging possibilities existing thus far, a possibility for controlling the direction of the shift ( 11 ) of parts of the object to be examined which exhibit deviantly resonating spins. In accordance with the invention the object is achieved by means of an MR imaging method of the kind set forth in which the orientation of at least one field strength gradient ( 13 ) of a gradient magnetic field along an axis of the magnet co-ordinate system is provided as a measuring parameter. An advantage of the invention is formed by the enhanced possibilities for imaging.

The invention relates to a method for the MR imaging of an object to beexamined by means of an MR apparatus which includes at least one mainmagnetic field device and at least one gradient magnetic field devicefor position encoding, in which a control algorithm controls measuringsequences in conformity with predetermined measuring parameters, inwhich the magnetic moment of the spins is aligned on average in apreferred direction by means of a main magnetic field, in which at leastone gradient magnetic field which traverses at least partly the objectto be examined and whose field strength exhibits a field strengthgradient which is at least partly essentially constant in at least onespatial direction is applied for a given period of time, in which thepreferred direction of the magnetic field generated by means of the mainmagnetic field device defines an axis of a stationary magneticco-ordinate system, in which at least one further axis of the magnetco-ordinate system is defined by the direction of the field strengthgradient of the magnetic field which can be generated by means of agradient magnetic field device, in which axes of a measuring co-ordinatesystem are defined each time by the direction of the slices and/or thedirection of the phase encoding and/or the reading direction on theobject to be examined, and in which a co-ordinate system is associatedwith the object to be examined.

An MR apparatus comprises essentially a strong magnet which generates asteady main magnetic field and a plurality of gradient coils whichgenerate a respective gradient magnetic field in each of the threespatial directions. Generally speaking, amplifiers are associated withthe gradient coils. Parts of the object to be examined are excited bymeans of an RF transmitter. The MR signal is picked up and amplified bymeans of a very sensitive RF receiver; during this operation anautomatic switch frequently makes the transmission coil also suitablefor the reception. Furthermore, an MR apparatus includes severalcomputers for executing the control algorithm, for controlling theapparatus, for reconstructing the MR images, for co-ordinating thevarious processes, notably during the measurement, for enabling entriesvia the control console and for filing the acquired images.

In the context of a typical magnetic resonance imaging method themagnetic moment of the protons is aligned in one spatial direction bymeans of a strong, steady magnetic field of from approximately 1.5 to 3Tesla Using brief electromagnetic RF pulses, the individual protons areexcited to precession and subsequently they are aligned again inconformity with the external strong magnetic field. Notably theexcitation and relaxation times as well as the frequencies of theprecessional motions are dependent on the tissue and in the context ofthe measurement they provide, in conjunction with the position encodingof the excitation, information on the situation in space of differenttissues. The position encoding utilizes position-dependent frequenciesand phases of the precessional excitation and offers, via Fouriertransformation of the measured MR signals, information as regards theposition of the relevant emission. In order to generate the magneticfield strength gradients desired for position encoding, generally use ismade of three different coil systems which extend in three mutuallyperpendicular spatial directions. The two spatial directions which areoriented perpendicularly to the longitudinal axis of the body aregenerally referred to as the x direction and the y direction.

Differently resonating spins cause a variety of artifacts in MR imaging.One such artifact is the chemical shift which is due to the fact thatprotons in fat tissue (hydrogen bound to carbon) and protons in water(hydrogen bound to oxygen) are subject to a magnetic field of differentstrength due to their different environments. Therefore, they have aresonance frequency which is slightly shifted relative to one another,the difference between their resonance frequencies increasing as thestrength of the outer main magnetic field is greater. For example, inthe case of spin echo sequences, this phenomenon becomes manifest in theMR images as a shift of the fat tissue towards the structures imaged onthe basis of water in the reading direction. After half the echo time,the spin echo sequence applies a 180° pulse which causes, after the fullecho time, the rephasing of the dephased spins and hence a strongsignal. However, a problem is encountered in that, if an acceptableexpenditure on time is to be achieved, the individual measuring valuesmust be acquired at different instants after the excitation pulse or the180° pulse, so that the undesirable artifacts may arise.

Extraordinarily resonating spins may also be due to inhomogeneities ofthe main magnetic field. This is the case notably when the tissue to beexamined locally exhibits different susceptibilities, for example, dueto metal implants or air/tissue interfaces so that the main magneticfield is no longer sufficiently homogeneous. The local variations of thesusceptibility cause undesirable local gradients of the main magneticfield. As a result, the frequency of the measured signal is shifted,such a shift being proportional to the local variations of the mainmagnetic field. Depending on the direction of the gradient of thegradient magnetic field and the gradient caused by the changedsusceptibility of the main magnetic field, the causal inhomogeneous partof the object to be examined is represented in the image formed in acompressed or expanded form. Compression of the representation of partsof the image formed is problematic notably because the structure of theregions shown is then usually distorted to such an extent that medicalinformation or a diagnosis can no longer be derived therefrom. Moreover,the compression leads to a local increase of the intensity of thesignal; this may give rise to bright spots in the image formed. Theseeffects are highly undesirable already because of the fact that they maygive rise to misinterpretation of the image and to incorrect diagnoses.

Spins resonating in a deviant manner also cause problems in respect ofthe excitation of individual slices, because the deviantly resonatingspins can also be excited outside the slice to be excited. Consequently,undesirable changes of contrast may arise in the image formed, becausethe signal emitted by the tissue containing water in some slices issuperposed on the signal from the tissue containing fat from neighboringslices. The demarcation of individual slices to be imaged relative toone another is substantially impeded by such artifacts.

In conventional MR methods the direction of such types of shift can becontrolled by a suitable choice of the scan technique, the orientationof the slices and the spatial direction of the phase encoding. Suchcontrol of the shift direction by one of these parameters, however, hasthe drawback that this parameter is no longer available for the furtherformation of the image. The scan technique in particular is dictatedalready by the necessary tissue contrast, the scan time and the desiredinsensitivity to changes of the susceptibility. In most cases theorientation of the slices is already governed by the relevant medicaltask and cannot be freely chosen either because of so-called fold-overproblems. Moreover, the phase encoding direction is alreadypredetermined in most cases in order to avoid fold-over artifacts.

Considering the drawbacks and problems of the present state of the art,it is an object of the invention to provide an MR imaging method wherebythe shift of parts of the object to be examined which exhibit deviatingresonating spins can be controlled in respect of direction withouthaving to sacrifice any of the known possibilities for forming the MRimage.

The object is achieved in accordance with the invention by means of anMR imaging method of the kind set forth in which the orientation of atleast one field strength gradient, generated during the measurement, ofa gradient magnetic field along an axis of the measuring co-ordinatesystem is provided as a measuring parameter for the method.

A special advantage of the invention resides in the fact that theoperator of the MR apparatus can freely choose the directions of theshift of the parts of the examination volume with deviantly resonatingspins, without having to sacrifice other possibilities for imaging. Theoperator can thus control at will the direction of the shift offat-containing tissue or the shifts caused by inhomogeneities of themain magnetic field.

In order to ensure that the operator of the MR apparatus need not dealwith excessive geometrical aspects, it is advantageous when theorientation of the field strength gradient of the gradient magneticfield is provided in the form of direction indications related to theco-ordinate system of the object to be examined. The operator can thusconcentrate better on the medical task.

In an advantageous further version of the invention the controlalgorithm asks the operator of the MR apparatus for the desireddirection of the shift of fat tissue in the image when the desireddirection of the field strength gradient is selected on the controlconsole. As a result, the operator of the MR apparatus, usually beingentrusted with medical tasks, will not be confronted with an abstracttask but will only be requested to answer a comprehensible question.Even though the decision taken by the operator influences not only theso-called “fat shift”, this approach facilitates the procedureconsiderably.

Another possibility for facilitating the selection of the desireddirection of the field strength gradient by the operator at the controlconsole, that is, notably in the case of excitation of individualslices, is given by the fact that the control algorithm requests theoperator at the control console to indicate the desired direction of theshift of the slice to be excited.

In order to facilitate the operation of the MR apparatus even furtherwhile using the method in accordance with the invention, it isadvantageous when, upon selection of the desired direction of the fieldstrength gradient, the control algorithm asks the operator of the MRapparatus for the desired direction of the shift of fat tissue in theimage along a predetermined axis of the co-ordinate system of the objectto be examined, so that the operator has to choose from twopossibilities only. Thus, the operator need not worry about thegeometrical circumstances without a mental support, that is, practicallywithout the possibilities of imaging being limited by this choice.

In order to ensure that a logical sequence of electromagnetic pulses andmagnetic fields is always obtained during the measurement, it makessense for the control algorithm to determine the relevant axes for theshift of the fat tissue in the image from the other parameters of theposition encoding, and to request from the operator as input therespective desired direction of the shift of fat tissue in the imageassociated with these axes.

In conformity with a version of the invention, the measuring method isgiven only the direction of the field strength gradient of a gradientmagnetic field device in direction indications related to theco-ordinate system of the object to be examined, that is, each timeprior to the measurement. This may be effective for given sequences andmay also facilitate the operation of the MR apparatus.

In order to achieve an as versatile as possible control of the MRapparatus it is arranged that the measuring method is given thedirections of the field strength gradients of all gradient magneticfield devices of the MR apparatus which are used in the correspondingimage, that is, in direction indications related to the co-ordinatesystem of the object to be examined and each time prior to themeasurement. All imaging possibilities in the sense of the invention arethus utilized.

For further simplification of the operation it is arranged in accordancewith the invention that the user receives on the control console eachtime a direction indication, related to the co-ordinate system of theobject to be examined, for the direction of the field strength gradientof the gradient magnetic field device. The proposal made to the user isdetermined by the control algorithm of the MR apparatus, notably fromthe indications of the scan technique, the slice direction, and thedirection of the phase encoding.

In order to ensure that the indications from the operator in respect ofthe shifting of the regions with deviantly resonating spins are alsocorrectly interpreted by the method, it is advantageous when the axes ofthe co-ordinate system of the object to be examined extend parallel tothe axes of the magnet co-ordinate system. This convention also holdswhen the object to be examined is arranged in the MR apparatus withpreferred directions which do not extend parallel to the axes of themagnet co-ordinate system.

The invention also relates to a magnetic resonance imaging systemcomprising

-   -   a receiver system to acquire magnetic resonance signals are        acquired from an object, the object to be examined having        several main axes, and    -   a gradient system to apply several encoding gradients to        spatially encode the magnetic resonance signals wherein    -   the gradient system is arranged to select the mutual orientation        of the encoding gradients on the basis of the mutual orientation        of the main axes of the object.

Because in the magnetic resonance imaging system of the invention thegradient system enables selection of the mutual orientation of theencoding gradients relative to the main axes of the object to beexamined, shift of imaged fat containing tissue or shifts in the imagecaused by inhomogeneities are more easily controlled. Further, theinvention relates to a computer program as defined in claim 11. Thecomputer program of the invention when installed in the working memoryof a processor of the magnetic resonance imaging system enables themagnetic resonance imaging system to perform the method of theinvention. The computer program of the invention can be provided on adata carrier such as a CD-rom, or can be downloaded from a data networksuch as the world-wide web.

The invention will be described in detail hereinafter with reference toa special embodiment and drawings. Therein:

FIG. 1 is a diagrammatic representation of the execution of the methodin accordance with the invention in the case of an echo planar sequence.

In FIG. 1 the individual steps of the method are denoted by the numbers1 to 4 in conformity with their order of execution. The invention willbe described on the basis of an example of an echo planar sequence.

In a first step 1 of the method, the operator selects the orientation inspace of the slice 6 on a control console (PC). The operator of the MRapparatus then has the three spatial axes available (in this caseright-left, anterior-posterior, feet-head; abbreviated as: RL, AP, FH).In conformity with the selected axis of the slice 6, a given slice of anobject 7 to be examined is excited in a step of the measurement in thata magnetic field having a given field strength gradient 13 in thedirection of the spatial axis selected for the slice 6 is applied acrossthe object 7 to be examined and individual slices are selectivelyexcited by means of a narrow-band RF pulse in conformity with the Larmorfrequency of the slice to be excited. The user in this case selects asagittal slice 6 (axis RL) from the three possibilities offered.

In a second step 2 of the method in accordance with the invention, theaxial direction (FH, RL, AP) is defined in which position encoding 8 isto be performed by means of the phase of the proton precession. Theaxial direction FH of the position encoding 8 preferably lies in theslice plane of the excited slice. The operator can now choose from twospatial directions (FH, AP). The position encoding 8 is performed byapplication of a magnetic field having a given field strength gradient13 in the direction of the phase encoding. In this case the axialdirection FH, extending in the FH direction (feet-head) is selected.

In a third step 3 of the method in accordance with the invention theoperator defines the orientation of the field strength gradient 13 alongthe axial direction FH determined in the second step. The controlalgorithm of the method in accordance with the invention offers theoperator two possibilities (parallel or anti-parallel along the axisdefined in the step 2) for the desired fat shift; the operator canchoose freely among these two possibilities. In this case the directionH on the FH axis is chosen, implying an anti-parallel gradient directionalong the FH axis, that is, in the direction F.

In a fourth step 4 the actual measurement is performed on the object 7to be examined. A part which contains fat in the position 12 of theobject 7 to be examined resonates in a manner which deviates from thatof the part containing water in the position 12 of the object to beexamined, that is, with a frequency which is approximately 3.5 ppm lower(frequency offset 17) than the parts containing water in the position 12of the object 7 to be examined. The signal 16 received from thefat-containing part in the position 12 is interpreted by the analysisunit of the MR apparatus in a manner as if the received signal 16 hasbeen emitted by a position situated further in the negative direction ofthe applied field strength gradient 13 (shift 11). The user hascontrolled the direction of the shift 11 of the fat-containing part inthe position 12 of the object 7 to be examined via the selection of theorientation of the field strength gradient 13 in the third step 3 of themethod in accordance with the invention.

1. A magnetic resonance imaging method in which magnetic resonancesignals are acquired from an object wherein several encoding gradientsare applied to spatially encode the magnetic resonance signals and theobject to be examined has several main axes and the mutual orientationof the encoding gradients is selected on the basis of the mutualorientation of the main axes of the object.
 2. An method for the MRimaging of an object to be examined by means of an MR apparatus whichincludes at least one main magnetic field device and at least onegradient magnetic field device for position encoding, In which a controlalgorithm controls measuring sequences in conformity with predeterminedmeasuring parameters, in which the magnetic moment of the spins isaligned on average in a preferred direction by means of a main magneticfield, in which at least one gradient magnetic field which traverses atleast partly the object to be examined and whose field strength exhibitsa field strength gradient which is at least partly essentially constantin at least one spatial direction is applied for a given period of time,in which the preferred direction of the magnetic field generated bymeans of the main magnetic field device defines an axis of a stationarymagnetic co-ordinate system, in which at least one further axis of themagnet co-ordinate system is defined by the direction of the fieldstrength gradient of the magnetic field which can be generated by meansof a gradient magnetic field device, in which axes of a measuringco-ordinate system are defined each time by the direction of the slicesand/or the direction of the phase encoding and/or the reading directionon the object to be examined, and in which a co-ordinate system isassociated with the object to be examined, wherein the orientation of atleast one field strength gradient, generated during the measurement, ofa gradient magnetic field along an axis of the measuring co-ordinatesystem is provided as a measuring parameter for the method.
 3. A methodas claimed in claim 2, wherein the orientation of the field strengthgradient of the gradient magnetic field is provided in the form ofdirection indications related to the co-ordinate system of the object tobe examined.
 4. A method as claimed in claim 2, wherein the controlalgorithm asks the operator of the MR apparatus for the desireddirection of the shift of fat tissue in the image when the desireddirection of the field strength gradient is selected on the controlconsole.
 5. A method as claimed in claim 4, wherein the desireddirection of the field strength gradient is selected on the controlconsole, the control algorithm asks the operator of the MR apparatus forthe desired direction of the shift of fat tissue in the image along apredetermined axis of the co-ordinate system of the object to beexamined, so that the operator has to choose among two possibilitiesonly.
 6. A method as claimed in claim 4, wherein the control algorithmdetermines the relevant axes for the shift of the fat tissue in theimage from the other parameters of the position encoding and requestsfrom the operator as input the respective desired direction of the shiftof fat tissue in the image associated with these axes.
 7. A method asclaimed in claim 1, wherein the measuring method is given only thedirection of the field strength gradient of a gradient magnetic fielddevice in direction indications related to the co-ordinate system of theobject to be examined, that is, each time prior to the measurement.
 8. Amethod as claimed in claim 1, wherein the measuring method is given thedirections of the field strength gradients of all gradient magneticfield devices of the MR apparatus which are used in the correspondingimage, that is, in direction indications related to the co-ordinatesystem of the object to be examined and each time prior to themeasurement.
 9. A method as claimed in claim 1, wherein the userreceives on the control console each time a direction indication,related to the co-ordinate system of the object to be examined, for thedirection of the field strength gradient of the gradient magnetic fielddevice.
 10. A method as claimed in claim 1, wherein the axes of theco-ordinate system of the object to be examined extend parallel to theaxes of the magnet co-ordinate system.
 11. A computer program comprisinginstructions to select the mutual orientation of encoding gradients onthe basis of the mutual orientation of the main axes of an object to beexamined.
 12. A magnetic resonance imaging system comprising a receiversystem to acquire magnetic resonance signals are acquired from anobject, the object to be examined having several main axes, and agradient system to apply several encoding gradients to spatially encodethe magnetic resonance signals wherein the gradient system is arrangedto select the mutual orientation of the encoding gradients on the basisof the mutual orientation of the main axes of the object.